The present invention relates generally to bimetal snap disc thermostats and more particularly to an improved bimetal snap disc thermostat in which resistance heaters are employed to depress the ambient temperatures at which such thermostats are actuated.
Bimetal snap disc thermostats which provide an electrical resistance heater controlled by an external control circuit to change the operating temperature of the thermostat are known. An example of such a thermostat is illustrated and described in U.S. Pat. No. 3,248,501 (assigned to the assignee of the present invention), which patent is incorporated herein by reference. Such device includes an annular disc-shaped heater of special construction which is positioned adjacent to the snap disc. Because the disc heater is not a standard available heating device of general utility, it is relatively expensive to produce. Further, when the disc heater is installed in the thermostat, it must be subsequently connected to the terminals. Consequently, the thermostat in accordance with such patent is relatively expensive to produce and is also relatively expensive to assemble.
It is also known to provide strip heaters in combination with blade-type bimetal thermostats, as illustrated in U.S. Pat. No. 3,870,985. Here again, the heater is of a special construction, and therefore relatively expensive to produce.
More recently, however, bimetal snap disc thermostats have been developed which utilize conventional commercially available resistance type heaters. The use of such resistance type heaters is disclosed in U.S. Pat. No. 4,533,894 (assigned to the assignee of the present invention).
Additionally, U.S. Pat. No. 5,576,683 discloses a bimetal snap disc thermostat utilizing resistance heaters in which the resistance heaters are supported closely adjacent a bimetal snap disc by a separate thermal insulator member. In this thermostat, a thin sheet film member is required to ensure the resistance heaters are electrically insulated from the snap disc. While this thermostat provided greater temperature depression than the prior art thermostats, it required the manufacture and assembly of both the thermal insulator support as well as the sheet film insulator which resulted in increased costs. Further, the sheet film also tends to slightly thermally insulate the bimetal snap disc from the resistance heaters thus limiting the effective temperature depression that can be achieved.
It should also be noted that the size of air volume of the chamber within which the resistance heaters are located may adversely effect the efficiency of the resistance heaters in depressing the response temperature of the bimetal snap disc.
In one application, these bimetal snap action switches are utilized to control temperatures in clothes dryers. In such applications, it is increasingly desirable to provide such thermostats with the ability to offer greater and greater temperature depression capability in order to offer a wider range of drying temperatures. In previous efforts to accommodate this increased temperature depression, higher wattage resistance heaters have been required but in some cases, the increase wattage of the heaters has required the use of more costly ceramic housings as opposed to the less expensive phenolic switch cases. Because ceramic switch cases are significantly more fragile than the phenolic counterparts, it has been difficult, if not impossible, to manufacture such thermostats in an automated assembly line. This aspect also significantly increases the cost of such thermostats.
The bimetal snap disc thermostat of the present invention overcomes these disadvantages by providing a chamber to accommodate the resistors having a smaller volume and providing locating tabs to aid in more precisely positioning of the resistor heaters thereby enabling the elimination of the sheet film insulator between the heaters and the bimetal snap disc while still allowing the heaters to be positioned within close proximity to the bimetal snap disc. Also, the resistor heaters and their associated leads are entirely suspended in the chamber by the terminals to which the leads are secured thus providing an insulating air layer between the resistor heater body including its leads and the phenolic switch case. All of these modifications contribute to more efficient heat transfer from the heating resistors to the bimetal snap disc thus allowing greater temperature depression with lower wattage heating resistors while also enabling the use of phenolic switch case without exceeding its thermal limits. Additionally, the raised locating tabs serve an additional function of further strengthening the switch case thus reducing the possibility of damage thereto when the snap disc retainer is crimped into position. Additionally, the switch case includes guide surfaces operative to assist in assembly of the heating resistors and associated contacts during assembly.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.